Liquid crystalline medium

ABSTRACT

The invention relates to a liquid crystalline medium, based on a mixture of polar compounds with a positive dielectric anisotropy. Said medium is characterised in that it contains one or more compounds of formula (I) wherein R, A 1 , A 2 , Z 1 , Z 2 , X 1 , X 2 , n and o are defined as per claim  1.

The present invention relates to a liquid-crystalline medium, to the usethereof for electro-optical purposes, to displays containing thismedium, and to novel compounds for use in the liquid-crystalline mediumaccording to the invention.

Liquid crystals are used principally as dielectrics in display devices,since the optical properties of such substances can be modified by anapplied voltage. Electro-optical devices based on liquid crystals areextremely well known to the person skilled in the art and can be basedon various effects. Examples of such devices are cells having dynamicscattering, DAP (“deformation of aligned phases”) cells, guest/hostcells, TN cells having a twisted nematic structure, STN (“supertwistednematic”) cells, SBE (“superbirefringence effect”) cells, OMI (“opticalmode interference”) cells, OCB (“optically compensated bend mode”) andIPS (“in-plane-switching”) cells.

The commonest display devices are based on the Schadt-Helfrich effectand have a twisted nematic structure, wie z.B. in TN— und STN-Zellen.Sie können als Multiplex-oder als Aktivmatrix-Anzeigen (AMD-TN, AMDactive matrix driven) betrieben werden.

In the case of TN displays, liquid-crystal media are desired whichenable the following advantages in the cells: an expanded nematic phaserange (in particular down to low temperatures), switchability atextremely low temperatures (outdoor use, automobiles, avionics) andincreased resistance to UV radiation (longer service life). With themedia available from the prior art, however, it is not possible toachieve these advantages while simultaneously retaining the aboveparameters.

In the case of the more highly twisted STN displays, liquid-crystalmedia are desired which enable greater multiplex ability and/or lowerthreshold voltages and/or broader nematic phase ranges (in particular atlow temperatures). To this end, a further extension of the availableparameter latitude (clearing point, smectic-nematic transition ormelting point, viscosity, dielectric parameters, elastic parameters) isurgently desired.

OCB displays contain a liquid-crystal layer having a so-called “bend”structure. The “bend” cell, also known as “π” cell, was proposed for thefirst time by P. Bos et al., SID 83 Digest, 30 (1983), for electricallycontrollable λ/s retardation plates. Optical displays based on the OCBcell have been described by Y. Yamaguchi, T. Miyashita and T. Uchida,SID 93 Digest, 277 (1993), T. Miyashita et al. in Proc. Eurodisplay, 149(1993), J. Appl. Phys. 34, L177 (1995), SID 95 Digest, 797 (1995), andC.-L. Kuo et al., SID 94 Digest, 927 (1994). OCB cells usually contain aliquid-crystal layer having a homogeneous edge alignment (i.e. parallelto the surfaces) and positive dielectric anisotropy. In addition, theOCB displays disclosed in the above-mentioned documents usually have oneor more optical retardation films in order to reduce undesired lighttransmission of the “bend” cell in the dark state. OCB displays havesome advantages over conventional TN cells, such as, for example, awider viewing angle and shorter response times.

OCB displays require liquid-crystalline media which have to satisfy amultiplicity of requirements. Particularly important here are thechemical resistance to moisture, air and physical influences, such asheat, radiation in the infrared, visible and ultraviolet region, anddirect and alternating electric fields. Furthermore, liquid-crystalmedia for the OCB effect which can be used industrially are required tohave a liquid-crystalline phase in a suitable temperature range,relatively high birefringence, positive dielectric, anisotropy, arelatively low value for the ratio of the elastic constants K₃/K₁ andlow viscosity. There is thus a great demand for liquid-crystalline mediafor OCB displays which exhibit, in particular, high values for thebirefringence and dielectric anisotropy and at the same time lowviscosities.

Besides the known liquid-crystal displays (TN, STN, OMI, AMD-TN andOCB), in which the electric fields for realignment are generatedessentially perpendicular to the liquid-crystal layer, displays alsoexist in which the electric signals are generated in such a way that theelectric fields have a significant component parallel to theliquid-crystal layer. A display of this type, known as an IPS (“in-planeswitching”) display, is disclosed, for example, in international patentapplication WO 91/10936. The principles, of operation of a display ofthis type are described, for example, by R. A. Soref in Journal ofApplied Physics, Vol. 45, No. 12, pp., 5466-5468 (1974). For example, EP0 588 568 discloses various possibilities for the design of theelectrodes and for addressing a display of this type. DE 198 24 137likewise describes various embodiments of IPS displays of this type.Liquid-crystalline materials for IPS displays are described, forexample, in DE 195 28 104.

The IPS displays containing the known liquid-crystalline media arecharacterised by inadequate, long response times and often byexcessively high operating voltages. There is thus a demand forliquid-crystal media for IPS displays which do not have thesedisadvantages or only do so to a lesser extent. To this end, there is aparticular requirement for liquid-crystalline materials which, besidesan adequate phase range, a low tendency toward crystallisation at lowtemperatures, low birefringence and adequate electrical resistance,have, in particular, low threshold voltages (V₁₀) and low rotationalviscosities (γ₁), which are crucial for the response times.

Just like TN displays, OCB and IPS displays can also be operated asmatrix displays.

Matrix liquid-crystal displays of this type are known. Non-linearelements which can be used for individual switching of the individualpixels are, for example, active elements (i.e. transistors). The term“active matrix” is then used, where a distinction can be made betweentwo types:

-   1. MOS (metal oxide semiconductor) or other diodes on a silicon    wafer as substrate.-   2. Thin-film transistors (TFTs) on a glass plate as substrate.

The use of single-crystal silicon as substrate material restricts thedisplay size, since even modular assembly of various part-displaysresults in problems at the joints.

In the case of the more promising type 2, which is preferred, theelectro-optical effect used is usually the TN effect. A distinction ismade between two technologies: TFTs comprising compound semiconductors,such as, for example, CdSe, or TFTs based on polycrystalline oramorphous silicon. The latter technology is being worked on intensivelyworldwide.

The TFT matrix is applied to the inside of one glass plate of thedisplay, while the other glass plate carries the transparentcounterelectrode on its inside. Compared with the size of the pixelelectrode, the TFT is very small and has virtually no adverse effect onthe image. This technology can also be extended to fully colour-capabledisplays, in which a mosaic of red, green and blue filters is generallyarranged in such a way that a filter element is opposite each switchablepixel.

The TFT displays usually operate as TN cells with crossed polarisers intransmission and are illuminated from the back. In the case of OCBdisplays, reflective displays have also been proposed, for example by T.Uchida, T. Ishinabe and M. Suzuki in SID 96 Digest, 618 (1996).

The term MLC displays here covers any matrix display with integratednon-linear elements, i.e., besides the active matrix, also displays withpassive elements, such as varistors or diodes(MIM=metal-insulator-metal).

For matrix liquid-crystal displays having integrated non-linear elementsfor switching individual pixels (MLC displays), liquid-crystal mediahaving large positive dielectric anisotropy, broad nematic phases,relatively low birefringence, very high specific resistance, good UV andtemperature stability and low vapour pressure are desired.

MLC displays are particularly suitable for TV applications (for examplepocket TVs) or for high-information displays for computer applications(laptops) and in automobile or aircraft construction. Besides problemsregarding the angle dependence of the contrast and the response times,difficulties also arise in MLC displays due to insufficiently highspecific resistance of the liquid-crystal mixtures [TOGASHI, S.,SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K., TAJIMA, E.,WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September 1984: A210-288 Matrix LCD Controlled by Double Stage Diode Rings, p. 141 ff,Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Design of ThinFilm Transistors for Matrix Addressing of Television Liquid CrystalDisplays, p. 145 ff, Paris]. With decreasing resistance, the contrast ofan MLC display deteriorates, and the problem of after-image eliminationmay occur. Since the specific resistance of the liquid-crystal mixturegenerally drops over the life of an MLC display owing to interactionwith the interior surfaces of the display, a high (initial) resistanceis very important in order to obtain acceptable service lives. Inparticular in the case of low-volt mixtures, it was hitherto impossibleto achieve very high specific resistance values. It is furthermoreimportant that the specific resistance exhibits the smallest possibledecrease with increasing temperature and after heating and/or UVexposure. The low-temperature properties of the mixtures from the priorart are also particularly disadvantageous. It is demanded that nocrystallisation and/or smectic phases occur, even at low temperatures,and the temperature dependence of the viscosity is as low as possible.The MLC displays from the prior art thus do not meet today'srequirements.

Besides liquid-crystal displays which use back lighting, i.e. areoperated transmissively and optionally transflectively, there is alsoparticular interest in reflective liquid-crystal displays. Thesereflective liquid-crystal displays use the ambient light for informationdisplay. They thus consume significantly less energy than back-litliquid-crystal displays of corresponding size and resolution. Since theTN effect is characterised by very good contrast, reflective displays ofthis type are readily legible even under bright ambient conditions. Thisis already known of simple reflective TN displays, as used, for example,in wristwatches and pocket calculators. However, the principle can alsobe applied to high-quality, higher-resolution active matrix-addresseddisplays, such as, for example, TFT displays. Here, as is already thecase in the generally conventional transmissive TFT-TN displays, the useof liquid crystals of low birefringence (Δn) is necessary in order toachieve low optical retardation (d·Δn). This low optical retardationresults in a low viewing-angle dependence of the contrast, which isusually acceptable (cf. DE 30 22 818). In reflective displays, the useof liquid crystals of low birefringence is much more important than intransmissive displays, since in reflective displays, the effective layerthickness through which the light passes is approximately twice as greatas in transmissive displays of the same layer thickness.

There thus continues to be a great demand for MLC displays having veryhigh specific resistance at the same time as a large working-temperaturerange, short response times even at low temperatures and low thresholdvoltage which do not have these disadvantages, or only do so to areduced extent.

In general, liquid-crystal materials for the above-mentioned displaytypes must have good chemical and thermal stability and good stabilitytowards electric fields and electromagnetic radiation. Furthermore, theliquid-crystal materials should have low viscosity and give shortaddressing times, low threshold voltages and high contrast in the cells.

Furthermore, they should have a suitable mesophase, for example anematic or cholesteric mesophase for the above-mentioned cells, at usualoperating temperatures, i.e. in the broadest possible range below andabove room temperature. Since liquid crystals are generally used in theform of mixtures of a plurality of components, it is important that thecomponents are readily miscible with one another. Other properties, suchas the electrical conductivity, the dielectric anisotropy and theoptical anisotropy, have to satisfy various requirements depending onthe cell type and area of application. For example, materials for cellshaving a twisted nematic structure should have positive dielectricanisotropy and low electrical conductivity.

None of the series of compounds having a liquid-crystalline phase thathave been disclosed hitherto contains a single compound which meets allthe above-mentioned requirements. In general, therefore, mixtures offrom 2 to 25, preferably from 3 to 18, compounds are prepared in orderto obtain substances which can be used as liquid-crystal phases.However, it has not been easy to prepare optimum phases in this way,since no liquid-crystal materials having high birefringence, positivedielectric anisotropy and high clearing point at the same time as lowrotational viscosity were available hitherto.

The invention has the object of providing liquid-crystalline media, inparticular for the above-mentioned MLC, TN, STN, OCB and IPS displays,which meet the above-mentioned requirements, do not have theabove-mentioned disadvantages, or only do so to a lesser extent, andpreferably at the same time have very high specific resistance valuesand low threshold voltages.

It has now been found that the above-described-objects can be achievedif media according to the invention are used in displays.

The invention thus relates to a liquid-crystalline medium based on amixture of polar compounds, characterised in that it comprises one ormore compounds of the formula I

in which

-   R is F, Cl, Br, I, CN, NCS, SF₅ or an alkyl radical having from 1,    to 12 carbon atoms which is unsubstituted, monosubstituted by CN or    CF₃ or monosubstituted or polysubstituted by halogen and in which    one or two non-adjacent CH₂ groups may be replaced by —O—, —CH═CH—,    —CF═CF—, —C≡C—, —CO—, —OCO—, or —COO— in such a way that O atoms are    not linked directly to one another,-   A¹ and A² are each, independently of one another, 1,4-phenylene, in    which, in addition, one or two CH groups may be replaced by N and    which may also be monosubstituted or polysubstituted by L, or are    trans-1,4-cyclohexylene, in which, in addition, one or more    non-adjacent CH₂ groups may be replaced by —O— and/or —S—, or are    1,4-cyclohexenylene, 1,4-bicyclo-[2.2.2]octylene,    piperidine-1,4-diyl, naphthalene-2,6-diyl,    decahydronaphthalene-2,6-diyl or    1,2,3,4-tetrahydronaphthalene-2,6-diyl,-   L is F, Cl, Br, I, CN, NCS, SF₅ or alkyl, alkoxy, alkylcarbonyl,    alkylcarbonyloxy, alkoxycarbonyl, alkenyl or oxaalkenyl having from    1 to 3 carbon atoms, in which, in addition, one or more H atoms may    be replaced by F or Cl,-   Z¹ and Z² are each, independently of one another, —CH₂O—, —OCH₂—,    —CF₂O—, —OCF₂—, —COO—, —OCO—, —CF₂CF₂—, —CH₂CH₂—, —(CH₂)₄—,    —(CH₂)₃O—, —O(CH₂)₃—, —CF₂CH₂—, —CH═CH—, —CH═CF—, —CF═CF—, —C≡C— or    a single bond,-   X¹ and X² are each, independently of one another, F, Cl, Br, I, CN,    NCS, SF₅ or alkyl, alkoxy, alkylcarbonyl, alkylcarbonyloxy,    alkoxycarbonyl, alkenyl or oxaalkenyl having up to 5 carbon atoms,    in which, in addition, one or more H atoms may be replaced by F or    Cl, and one of the radicals X¹ and X² is alternatively H or R.-   n and o are each, independently of one another, 0, 1 or 2, where n+o    is ≦3.

The compounds of the formula I have low rotational viscosities at thesame time as favourable clearing points and high dielectric anisotropyΔε, and effect a reduction in the threshold voltage in the mediaaccording to the invention at the same time as an optimisation of thelow-temperature behaviour. They are particularly suitable for use inTFT-TN displays with all common operating voltages (driver voltage of 5V, 4 V, 3.3 V and 2.5 V), in particular for TFT-TN displays having lowthreshold voltages (driver voltage of 2.5 and 3.3 V), and, owing totheir favourable birefringence values, for TN displays at the firstGooch and Tarry transmission minimum [C. H. Gooch and H. A. Tarry,Electron. Left. 10, 2-4, 1974, and Appl. Phys., Vol. 8, 1575-1584,1975].

The compounds of the formula I have a broad range of applications. Thesecompounds can be used as base materials of which liquid-crystallinemedia are predominantly composed; however, it is also possible to addcompounds of the formula I to liquid-crystalline base materials fromother classes of compound in order, for example, to modify thedielectric and/or optical anisotropy of a dielectric of this type and/orto optimise its threshold voltage and/or its viscosity.

In the pure state, the compounds of the formula I are colourless andform liquid-crystalline mesophases in a temperature range which isfavourably located for electro-optical use. They are stable chemically,thermally and to light. The compounds of the formula I are prepared bymethods known per se, as described in the literature (for example in thestandard works, such as Houben-Weyl, Methoden der Organischen Chemie[Methods of Organic Chemistry], Georg-Thieme-Verlag, Stuttgart), to beprecise under reaction conditions which are known and suitable for thesaid reactions. Use can also be made here of variants which are knownper se, but are not mentioned here in greater detail. Some compounds ofthe formula I and their preparation are disclosed in DE 43 03 634 and DE44 09 526. The novel compounds of the formula I are a furthersubject-matter of the invention.

Particular preference is given to compounds of the formula I in whichn+o is 0 or 1.

Preference is furthermore given to compounds of the formula I and theirsub-formulae in which one or both radicals X¹ and X² are F, Cl, CN orfluorinated alkyl or alkoxy having from 1 to 3 carbon atoms, inparticular CF₃, OCF₃, CF₂H or OCF₂H, furthermore OCH₃ or OC₂H₅.

Preference is furthermore given to compounds of the formula I and theirsub-formulae in which Z¹ and Z² are —OCO—, —COO—, —CF₂O—, —OCF₂—,—CH₂CH₂—, —CF₂CF₂— or a single bond.

Preference is furthermore given to compounds of the formula I and theirsub-formulae in which R is alkyl or alkoxy having from 1 to 8 carbonatoms.

The compounds of the following sub-formulae are particularly preferred.In these, Cyc denotes a 1,4-cyclohexylene radical, which may also besubstituted in the 1- and/or 4-position by F, Cl, CN or CF₃, Dio denotesa 1,3-dioxane-2,5-diyl radical, Phe denotes a 1,4-phenylene radical,which may be substituted in the 2-, 3- and/or 5-position by L. Z has oneof the meanings indicated for Z¹ in the formula I. InX¹X² denotes anindan-2-yl radical in accordance with the formula I which is substitutedin the 5-position by X¹ and in the 6-position by X². L, R, X¹ and X² areas defined in the formula I.R-Cyc-Z¹-InX¹X²  I1R-Phe-Z¹-InX¹X²  I2R-Dio-Z¹-InX¹X²  I3R-Cyc-Z²-Phe-Z¹-InX¹X²  I4R-Cyc-Z²-Cyc-Z¹-InX¹X²  I5R-Phe-Z²-Cyc-Z¹-InX¹X²  I6R-Phe-Z²-Phe-Z¹-InX¹X²  I7R-Cyc-Z²-Dio-Z¹-InX¹X²  I8R-Dio-Z²-Cyc-Z¹-InX¹X²  I9R-Dio-Z²-Dio-Z¹-InX¹X²  I10R-Phe-Z²-Dio-Z¹-InX¹X²  I11R-Dio-Z²-Phe-Z¹-InX¹X²  I12R-Cyc-Z²-Cyc-Z¹-Cyc-Z¹-InX¹X²  I13R-Cyc-Z²-Cyc-Z¹-Phe-Z¹-InX¹X²  I14R-Phe-Z²-Cyc-Z¹-Cyc-Z¹-InX¹X²  I15R-Cyc-Z²-Phe-Z¹-Phe-Z¹-InX¹X²  I16R-Phe-Z²-Phe-Z¹-Cyc-Z¹-InX¹X²  I17R-Phe-Z²-Phe-Z¹-Phe-Z¹-InX¹X²  I18R-Cyc-Z²-Cyc-Z¹-Dio-Z¹-InX¹X²  I19R-Dio-Z²-Cyc-Z¹-Cyc-Z¹-InX¹X²  I20R-Cyc-Z²-Dio-Z¹-Dio-Z¹-InX¹X²  I21R-Dio-Z²-Dio-Z¹-Cyc-Z¹-InX¹X²  I22R-Dio-Z²-Dio-Z¹-Dio-Z¹-InX¹X²  I23R-Dio-Z²-Dio-Z¹-Phe-Z¹-InX¹X²  I24R-Dio-Z²-Phe-Z¹-Phe-Z¹-InX¹X²  I25 R-InX¹X²  I26

Particular preference is given to compounds of the sub-formulae I1, I2,I4, I5, I6 and I7 and I26.

Very particularly preferred compounds of the formula I are selected fromthe following sub-formulae Ia to Ie:

in which R, Z¹, Z², X¹ and X² are as defined in the formula I.

Particular preference is given to compounds of the formula Ia, Ib andId, in particular those in which X¹ and X² are F, Z¹ is a single bondand R is alkyl or alkoxy having from 1 to 8 carbon atoms.

Preference is furthermore given to compounds of the formulae I and Ia toId in which

-   -   X¹ and X² are F, Cl, CN, CF₃, OCF₃, CF₂H or OCF₂H,    -   one of the radicals X¹ and X² is F, CN, C₁, CF₃ or OCF₃ and the        other is H or alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl or        alkylcarbonyloxy having from 1 to 8 carbon atoms,    -   Z¹ and Z² are a single bond,    -   one or both radicals Z¹ and Z² are —CF₂O—, —OCF₂— or —CF₂CF₂—,    -   R is alkyl or alkoxy having from 1 to 8 carbon atoms.

The term “fluorinated alkyl or alkoxy having from 1 to 3 carbon atoms”is preferably CF₃, OCF₃, CFH₂, OCFH₂, CF₂H, OCF₂H, C₂F₅, OC₂F₅, CFHCF₃,CFHCF₂H, CFHCFH₂, CH₂CF₃, CH₂CF₂H, CH₂CFH₂, CF₂CF₂H, CF₂CFH₂, OCFHCF₃,OCFHCF₂H, OCFHCFH₂, OCH₂CF₃, OCH₂CF₂H, OCH₂CFH₂, OCF₂CF₂H, OCF₂CFH₂,C₃F₇ or OC₃F₇, in particular CF₃, OCF₃, CF₂H, OCF₂H, C₂F₅, OC₂F₅,CFHCF₃, CFHCF₂H, CFHCFH₂, CF₂CF₂H, CF₂CFH₂, OCFHCF₃, OCFHCF₂H, OCFHCFH₂,OCF₂CF₂H, OCF₂CFH₂, C₃F₇ or OC₃F₁, particularly preferably OCF₃ orOCF₂H.

Halogen is preferably F or Cl, in particular F.

If R is an alkyl radical and/or an alkoxy radical, this may bestraight-chain or branched. It is preferably straight-chain, has 2, 3,4, 5, 6 or 7 carbon atoms and accordingly is preferably ethyl, propyl,butyl, pentyl, hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy, hexyloxyor heptyloxy, furthermore methyl, octyl, nonyl, decyl, undecyl, dodecyl,tridecyl, tetradecyl, pentadecyl, methoxy, octyloxy, nonyloxy, decyloxy,undecyloxy, dodecyloxy, tridecyloxy or tetradecyloxy.

Oxaalkyl is preferably straight-chain 2-oxapropyl (=methoxymethyl),2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3- or4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl,2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl,or 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl.

If R is an alkyl radical in which one CH₂ group has been replaced by—CH═CH—, this may be straight-chain or branched. It is preferablystraight-chain and has from 2 to 10 carbon atoms. Accordingly, it is inparticular vinyl, prop-1- or -2-enyl, but-1-, -2- or -3-enyl; pent-1-,-2-, -3- or 4-enyl, hex-1-, -2-, -3-, 4- or -5-enyl, hept-1-, -2-, -3-,4-, -5- or 4-enyl, oct-1-, -2-, -3-, 4-, -5-, -6- or -7-enyl, non-1-,-2-, -3-, 4-, -5-, -6-, -7- or -8-enyl, or dec-1-, -2-, -3-, 4-, -5-,-6-, -7-, -8- or -9-enyl.

If R is an alkyl radical in which one CH₂ group has been replaced by —O—and one has been replaced by —CO—, these are preferably adjacent. Thesethus contain an acyloxy group —CO—O— or an oxycarbonyl group —O—CO.These are preferably straight chain and have from 2 to 6 carbon atoms.Accordingly, they are in particular acetoxy, propionyloxy, butyryloxy,pentanoyloxy, hexanoyloxy, acetoxymethyl, propionyloxymethyl,butyryloxymethyl, pentanoyloxymethyl, 2-acetoxyethyl,2-propionyloxyethyl, 2-butyryloxyethyl, 3-acetoxypropyl,3-propionyloxypropyl, 4-acetoxybutyl, methoxycarbonyl, ethoxycarbonyl,propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl,ethoxycarbonylmethyl, propoxycarbonylmethyl, butoxycarbonylmethyl,2-(methoxycarbonyl)ethyl, 2-(ethoxycarbonyl)ethyl,2-(propoxycarbonyl)ethyl, 3-(methoxycarbonyl)-propyl,3-(ethoxycarbonyl)propyl or 4-(methoxycarbonyl)butyl.

If R is an alkyl radical in which one CH₂ group has been replaced byunsubstituted or substituted —CH═CH— and an adjacent CH₂ group has beenreplaced by CO or CO—O or O—CO, this may be straight-chain or branched.It is preferably straight-chain and has from 4 to 13 carbon atoms.Accordingly, it is in particular acryloyloxymethyl, 2-acryloyloxyethyl,3-acryloyloxypropyl, 4-acryloyloxybutyl, 5-acryloyloxypentyl,6-acryloyloxyhexyl, 7-acryloyloxyheptyt, 8-acryloyloxybctyl,9-acryloyloxynonyl, 10-acryloyloxydecyl, methacryoyloxymethyl,2-methacryloyloxyethyl, 3-methacryloyloxypropyl, 4-methacryloyloxybutyl,5-methacryloyloxypentyl, 6-methacryloyloxyhexyl,7-methacryloyloxyheptyl, 8-methacryloyloxyoctyl; or9-methacryloyloxynonyl.

If R is an alkyl or alkenyl radical which is monosubstituted by CN orCF₃, this radical is preferably straight-chain. The substitution by CNor CF₃ is in any desired position.

If R is an alkyl or alkenyl radical which is at least monosubstituted byhalogen, this radical is preferably straight-chain, and halogen ispreferably F or Cl. In the case of polysubstitution, halogen ispreferably F. The resultant radicals also include perfluorinatedradicals. In the case of monosubstitution, the fluorine or chlorinesubstituent may be in any desired position, but is preferably in theco-position.

Compounds of the formula I which have wing groups R which are suitablefor polymerisation reactions are suitable for the preparation ofliquid-crystalline polymers.

Compounds containing branched wing groups R may occasionally be ofimportance owing to better solubility in the conventionalliquid-crystalline base materials, but in particular as chiral dopantsif they are optically active. Smectic compounds of this type aresuitable as components of ferroelectric materials.

Compounds of the formula I having SA phases are suitable for thermallyaddressed displays.

Branched groups of this type generally contain not more than one chainbranch. Preferred branched radicals R are isopropyl, 2-butyl(=1-methylpropyl), isobutyl (=2-methylpropyl), 2-methylbutyl, isopentyl(=3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl,2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy,3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexyloxy,1-methylhexyloxy and 1-methylheptyloxy.

If R is an alkyl radical in which two or more CH₂ groups have beenreplaced by —O— and/or —CO—O—, this may be straight-chain or branched.It is preferably branched and has from 3 to 12 carbon atoms.Accordingly, it is in particular biscarboxymethyl, 2,2-biscarboxyethyl,3,3-biscarboxypropyl, 4,4-biscarboxybutyl, 5,5-biscarboxypentyl,6,6-biscarboxyhexyl, 7,7-biscarboxyheptyl, 8,8-biscarboxyoctyl,9,9-biscarboxynonyl, 10,10-biscarboxydecyl, bis(methoxycarbonyl)methyl,2,2-bis(methoxycarbonyl)-ethyl, 3,3-bis(methoxycarbonyl)propyl,4,4-bis(methoxycarbonyl)butyl, 5,5-bis(methoxycarbonyl)pentyl,6,6-bis(methoxycarbonyl)hexyl, 7,7-bis-(methoxycarbonyl)heptyl,8,8-bis(methoxycarbonyl)octyl, bis(ethoxycarbonyl)methyl,2,2-bis(ethoxycarbonyl)ethyl, 3,3-bis(ethoxycarbonyl)-propyl,4,4-bis(ethoxycarbonyl)butyl or 5,5-bis(ethoxycarbonyl)hexyl.

The invention also relates to electro-optical displays containingliquid-crystal media according to the invention, in particular TN, STN,OCB, IPS or MLC displays having two plane-parallel outer plates, which,together with a frame, form a cell, integrated non-linear elements forswitching individual pixels on the outer plates, and a nematicliquid-crystal mixture of positive dielectric anisotropy and highspecific resistance which is located in the cell. The inventionfurthermore relates to the use of the liquid-crystal media according tothe invention for electro-optical use.

The liquid-crystal mixtures according to the invention enable asignificant widening of the available parameter latitude. The achievablecombinations of clearing point, viscosity at low temperature, thermaland UV stability and dielectric anisotropy are thus far superior toprevious materials from the prior art.

The requirement for a high clearing point, a nematic phase at lowtemperature, high Δε and at the same time low viscosity has hithertoonly been achieved to an inadequate extent. Although mixtures knownhitherto have relatively high values for the clearing point and for Δεas well as favourable birefringence, they still have inadequately lowvalues for the rotational viscosity γ₁.

Other mixture systems have comparable viscosities and Δε values, butonly have clearing points in the region of 60° C.

The liquid-crystal mixtures according to the invention, while retainingthe nematic phase down to −20° C. and preferably down to −30° C.,particularly preferably down to −40° C., enable clearing points above75° C., preferably above 80° C., particularly preferably above 85° C.,simultaneously dielectric anisotropy values Δε of ≧5, preferably ≧7, anda high value for the specific resistance to be achieved, enablingexcellent STN and MLC displays to be obtained. In particular, themixtures are characterised by low operating voltages. The TN thresholdsare <2.0 V, preferably 1.8 V, particularly preferably <1.6 V, veryparticularly preferably <1.4 V.

It goes without saying that, through a suitable choice of the componentsof the mixtures according to the invention, it is also possible forhigher clearing points (for example above 110°) to be achieved at ahigher threshold voltage or lower clearing points to be achieved atlower threshold voltages with retention of the other advantageousproperties. At viscosities correspondingly increased only slightly, itis likewise possible to obtain mixtures having greater Δε and thus lowerthresholds. The MLC displays according to the invention preferablyoperate at the first Gooch and Tarry transmission minimum [C. H. Goochand H. A. Tarry, Electron. Left. 10, 2-4, 1974; C. H. Gooch and H. A.Tarry, Appl. Phys., Vol. 8, 1575-1584, 1975] are used, where, besidesparticularly favourable electro-optical properties, such as, forexample, high steepness of the characteristic line and low angledependence of the contrast (German Patent 30 22 818), a lower dielectricanisotropy is sufficient at the same threshold voltage as in ananalogous display at the second minimum. This enables significantlyhigher specific resistances to be achieved using the mixtures accordingto the invention at the first minimum than in the case of mixturescomprising cyano compounds. Through a suitable choice of the individualcomponents and their proportions by weight, the person skilled in theart is able to set the birefringence necessary for a pre-specified layerthickness of the MLC display using simple routine methods.

The rotational viscosity γ₁ at 20° C. is preferably <160 mPa·s,particularly preferably <140 mpa·s. The nematic phase range ispreferably at least 90°, in particular at least 100°. This rangepreferably extends at least from −20° to +80.

Measurements of the capacity holding ratio (HR) [S. Matsumoto et al.,Liquid Crystals 5, 1320 (1989); K. Niwa et al., Proc. SID Conference,San Francisco, June 1984, p. 304 (1984); G. Weber et al., LiquidCrystals 5, 1381 (1989)] have shown that mixtures according to theinvention comprising compounds of the formula I exhibit a significantlysmaller decrease in the HR with increasing temperature than, forexample, analogous mixtures comprising cyanophenylcyclohexanes of theformula

or esters of the formula R

instead of the compounds of the formula I.

In addition, it has been found that mixtures according to the inventioncomprising compounds of the formula I have a higher clearing point andhigher Δε than analogous mixtures comprising cyanophenylcyclohexanes ofthe above-mentioned formula. Compared with the last-mentioned mixtures,the mixtures according to the invention also have a smaller Δn, which isadvantageous for many applications, in particular reflective andtransflective applications.

The UV stability of the mixtures according to the invention is alsoconsiderably better, i.e. they exhibit a significantly smaller decreasein the HR on exposure to UV.

The proportion of the compounds of the formula I in the mixture of themedia according to the invention as a whole is preferably 2-55%,preferably 3-35% and particularly preferably 5-15%.

The individual compounds of the following formulae and theirsub-formulae which can be used in the media according to the inventionare either known or can be prepared analogously to the known compounds.

Preferred embodiments are indicated below:

-   -   The medium additionally comprises one or more compounds selected        from the group consisting of the general formulae II to VII:    -    in which the individual radicals have the following meanings:    -   R⁰ is n-alkyl, alkoxy, fluoroalkyl, alkenyl or oxaalkenyl, each        having up to 9 carbon atoms,    -   Z³ is —COO—, —CF₂O—, —C₂F₄— or a single bond,    -   Z⁴ is —COO, —CF₂O—, —C₂F₄— or —C₂H₄—,    -   X⁰ is F, Cl, halogenated alkyl, alkenyl or alkoxy having up to 6        carbon atoms,    -   Y¹ and Y² are each, independently of one another, H or F, and    -   r is 0 or 1.    -   The compounds of the formula II are preferably selected from the        following group:    -    in which R⁰ and X⁰ are as defined above, R⁰ is particularly        preferably n-alkyl having from 1 to 8 carbon atoms or alkenyl        having from 2 to 7 carbon atoms, and X⁰ is particularly        preferably F, Cl, CF₃, OCF₃ or OCHF₂;    -   The compounds of the formula IV are preferably selected from the        following group:    -    in which R⁰, X⁰ and Y¹ are as defined above. R⁰ is particularly        preferably n-alkyl having from 1 to 8 carbon atoms or alkenyl        having from 2 to 7 carbon atoms. X⁰ is particularly preferably        OCF₃, OCHF₂ or F. Particular preference is given to compounds of        the formulae IVa, IVb, IVc, IVd and IVp;    -   The compounds of the formula VI are preferably selected from the        following group:    -    in which R⁰ and X⁰ are as defined above. R⁰ is particularly        preferably n-alkyl having from 1 to 8 carbon atoms or alkenyl        having from 2 to 7 carbon atoms. X⁰ is preferably OCF₃, OCHF₂        or F. Particular preference is given to compounds of the        formulae VIa, VIb, VIc, VIh, VIi and VIk;    -   The compounds of the formula VII are preferably selected from        the following group:    -    in which R⁰ and X⁰ are as defined above. R⁰ is particularly        preferably n-alkyl having from 1 to 8 carbon atoms or alkenyl        having from 2 to 7 carbon atoms. X⁰ is preferably OCF₃ or F,        particularly preferably F. Particular preference is given to the        compounds of the formulae VIIa and VIIb;    -   The medium additionally comprises one or more compounds selected        from the following group:    -    in which R⁰, X⁰, Y¹ and Y² each, independently of one another,        have one of the meanings indicated above. Y³ is H or F, and X⁰        is preferably F, Cl, CF₃, OCF₃, OCHF₂, alkyl, oxaalkyl,        fluoroalkyl or alkenyl, each having up to 6 carbon atoms.    -   Particular preference is given to compounds of the formula VIII,        in particular those in which Y¹, Y², Y³ and X⁰ are F.    -   The medium comprises further compounds, preferably selected from        the following group:    -    in which R⁰ and X⁰ are as defined above, and the 1,4-phenylene        rings may be substituted by CN, chlorine or fluorine. The        1,4-phenylene rings are preferably monosubstituted or        polysubstituted by fluorine atoms. In the compound XVIII, X⁰ is        particularly preferably F or Cl.    -   The medium additionally comprises one or more compounds selected        from the following group:    -    in which X³ has one of the meanings of X⁰ or is CN or NCS, and        R⁰, X⁰, Y¹, Y² and r are as defined above.    -   Particularly preferred sub-formulae from the group consisting of        the compounds of the formulae XIX and XX are the following:    -    in which R⁰ is as defined above and is particularly preferably        n-alkyl having from 1 to 8 carbon atoms or alkenyl having from 2        to 7 carbon atoms.    -   The medium preferably comprises one or more dioxanes of the        formulae D1 and D2:    -    in which R⁰ is as defined above.    -   The medium additionally comprises one or more compounds selected        from the following group:    -    in which R⁰ and Y¹ are as defined above, and R¹ and R² are        each, independently of one another, alkyl or alkoxy having from        1 to 8 carbon atoms or alkenyl having from 2 to 7 carbon atoms.    -   In the compounds of the formulae XXII to XXVII, R¹ and R² are        preferably alkyl or alkoxy having from 1 to 8 carbon atoms.    -   Particular preference is given to the compounds selected from        the following group:    -    in which R^(1a) and R^(2a) are each, independently of one        another, H, CH₃, C₂H₅ or n—C₃H₇, alkyl is an alkyl group having        from 1 to 7 carbon atoms, s is 0 or 1 and L is H, or F.    -   R⁰ is straight-chain alkyl or alkenyl having from 2 to 7 carbon        atoms;    -   The medium comprises compounds selected from the formulae II,        III, IV, V, VI, VII, VIII, XIX, XX, XXI, XXII, XIII, XXIV and        XXVII;    -   The medium comprises one or more compounds selected from the        formulae IIa, IIb, IVa, IVb, IVc, IVd and VIk in which X⁰ is F,    -   The medium comprises one or more compounds selected from the        formulae IIa, IVp, VIh and VIi in which X⁰ is OCF₃ or OCHF₂,    -   The I: (II+III+IV+V+VI+VII) weight ratio is preferably from 1:10        to 10:1;    -   The medium essentially consists of compounds selected from the        group consisting of the general formulae I to VIII and XIX to        XXIV;    -   The medium comprises one, two or three compounds of the formula        1, preferably selected from the formulae Ia and Ic;    -   The proportion of compounds of the formula I in the mixture as a        whole is from 2 to 55% by weight, in particular from 3 to 35% by        weight, very particularly preferably from 5 to 15% by weight;    -   The proportion of compounds of the formulae I to VII together in        the mixture as a whole is at least 50% by weight;    -   The proportion of compounds of the formulae II to VII and XIX to        XXVII in the mixture as a whole is from 30 to 95% by weight.

It has been found that even a relatively small proportion of compoundsof the formula I mixed with conventional liquid-crystal materials, butin particular with one or more compounds of the formulae II, III, IV, V,VI and/or VII, results in a significant lowering of the thresholdvoltage and in low birefringence values, with broad nematic phases withlow smectic-nematic transition temperatures being observed at the sametime, improving the shelf life. Particular preference is given tomixtures which, besides one or more compounds of the formula I, compriseone or more compounds of the formula IV, in particular compounds of theformula IVa, IVb, IVc and IVd in which X⁰ is F or OCF₃. The compounds ofthe formulae I to VII are colourless, stable and readily miscible withone another and with other liquid-crystalline materials.

The term “alkyl” covers straight-chain and branched alkyl groups having1-7 carbon atoms, in particular the straight-chain groups methyl, ethyl,propyl, butyl, pentyl, hexyl and heptyl. Groups having 2-5 carbon atomsare generally preferred.

The term “alkenyl” covers straight-chain and branched alkenyl groupshaving 0.2-7 carbon atoms, in particular the straight-chain groups.Particular alkenyl groups are C₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl,C₅-C₇-4-alkenyl, C₆-C₇-5-alkenyl and C₇-6-alkenyl, in particularC₂-C₇-1E-alkenyl, C₄-C₇ -3E-alkenyl and C₅-C₇-4-alkenyl. Examples ofpreferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl,1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl,3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl,4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5carbon atoms are generally preferred.

The term “fluoroalkyl” preferably covers straight-chain groups having aterminal fluorine, i.e. fluoromethyl, 2-fluoroethyl, 3-fluoropropyl,4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and 7-fluoroheptyl.However, other positions of the fluorine are not excluded.

The term “oxaalkyl” preferably covers straight-chain radicals of theformula C_(n)H_(2n+1)—O—(CH₂)_(m), in which n and m are each,independently of one another, from 1 to 6. n is preferably=1 and m ispreferably from 1 to 6.

Through a suitable choice of the meanings of R⁰ and X⁰, the addressingtimes, the threshold voltage, the steepness of the transmissioncharacteristic lines, etc., can be modified in the desired manner. Forexample, 1 E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxyradicals and the like generally result in shorter addressing times,improved nematic tendencies and a higher ratio of the elastic constantsK₃ (bend) and K₁ (splay) compared with alkyl or alkoxy radicals.4-alkenyl radicals, 3-alkenyl radicals and the like generally give lowerthreshold voltages and smaller values of K₃/K₁ compared with alkyl andalkoxy radicals.

A —CH₂CH₂ group generally results in higher values of K₃/K₁ comparedwith a single covalent bond. Higher values of K₃/K₁ facilitate, forexample, flatter transmission characteristic lines in TN cells with a90° twist (in order to achieve grey shades) and steeper transmissioncharacteristic lines in STN, SBE and OMI cells (greatermultiplexability), and vice versa.

The optimum mixing ratio of the compounds of the formulae I andII+III+IV+V+VI+VII depends substantially on the desired properties, onthe choice of the components of the formulae 1, II, III, IV, V, VIand/or VII, and the choice of any other components that may be present.Suitable mixing ratios within the range given above can easily bedetermined from case to case.

The total amount of compounds of the formulae I to XXVII in the mixturesaccording to the invention is not crucial. The mixtures can thereforecomprise one or more further components for the purposes of optimisingvarious properties. However, the observed effect on the addressing timesand the threshold voltage is generally greater, the higher the totalconcentration of compounds of the formulae I to XXVII.

In a particularly preferred embodiment, the media according to theinvention comprise compounds of the formulae II to VII (preferably II,III and/or IV, in particular IVa) in which X⁰ is F, OCF₃, OCHF₂, F,OCH═CF₂, OCF═CF₂ or OCF₂—CF₂H. A favourable synergistic effect with thecompounds of the formula I results in particularly advantageousproperties. In particular, mixtures comprising compounds of the formulaI and of the formula IVa, IVb, IVc and IVd are distinguished by theirlow threshold voltages.

The construction of the MLC display according to the invention frompolarisers, electrode base plates and surface-treated electrodescorresponds to the conventional construction for displays of this type.The term, “conventional construction” is broadly drawn here and alsocovers all derivatives and modifications of the MLC display, inparticular including matrix display elements based on poly-Si TFT orMIM.

A significant difference between the displays according to the inventionand the conventional displays based on the twisted nematic cellconsists, however, in the choice of the liquid-crystal parameters of theliquid-crystal layer.

The liquid-crystal mixtures which can be used in accordance with theinvention are prepared in a manner conventional per se. In general, thedesired amount of the components used in the lesser amount is dissolvedin the components making up the principal constituent, advantageously atelevated temperature. It is also possible to mix solutions of thecomponents in an organic solvent, for example in acetone, chloroform ormethanol, and to remove the solvent again, for example by distillation,after thorough mixing.

The dielectrics may also comprise further additives known to the personskilled in the art and described in the literature. For example, 0-15%of pleochroic dyes or chiral dopants can be added.

In the following examples, C denotes a crystalline phase, S a smecticphase, S_(C) a smectic C phase, N a nematic phase and I the isotropicphase.

V₁₀ denotes the voltage for 10% transmission (viewing angleperpendicular to the plate surface). t_(on) denotes the switch-on timeand t_(off) the switch-off time at an operating voltage corresponding to2 times the value of V₁₀. Δn denotes the optical anisotropy and n_(o)the refractive index. Δε denotes the; dielectric anisotropy(Δε=ε_(∥)-ε_(⊥), where ε₈₁ denotes the dielectric constant parallel tothe longitudinal molecular axes and ε_(⊥) denotes the dielectricconstant perpendicular thereto). The electro-optical data were measuredin a TN cell at the 1st minimum (i.e. at a d·Δn value of 0.5) at 20° C.,unless expressly stated otherwise. γ₁ denotes the rotational viscosity.

The above-mentioned data were measured at 20° C., unless expresslystated otherwise.

In the present application and in the examples below, the structures ofthe liquid-crystal compounds are indicated by means of acronyms, thetransformation into chemical formulae taking place in accordance withTables A and B below. All radicals C_(n)H_(2n+1) and C_(m)H_(2m+1), arestraight-chain alkyl radicals having n and m carbon atoms respectively.The coding in Table B is self-evident. In Table A, only the acronym forthe parent structure is indicated. In individual cases, the acronym forthe parent structure is followed, separated by a dash, by a code for thesubstituents R¹, R², L¹ and L²:

Code for R¹, R², L¹, L² R¹ R² L¹ L² nm C_(n)H_(2n+1) C_(m)H_(2m+1) H HnOm C_(n)H_(2n+1) OC_(m)H_(2m+1) H H nO.m OC_(n)H_(2n+1) C_(m)H_(2m+1) HH n C_(n)H_(2n+1) CN H H nN.F C_(n)H_(2n+1) CN H F nF C_(n)H_(2n+1) F HH nOF OC_(n)H_(2n+1) F H H nCl C_(n)H_(2n+1) Cl H H nF.F C_(n)H_(2n+1) FH F nF.F.F C_(n)H_(2n+1) F F F nCF₃ C_(n)H_(2n+1) CF₃ H H nOCF₃C_(n)H_(2n+1) OCF₃ H H nOCF₂ C_(n)H_(2n+1) OCHF₂ H H nS C_(n)H_(2n+1)NCS H H rVsN C_(r)H_(2r+1)—CH═CH—C_(s)H_(2s)— CN H H rEsNC_(r)H_(2r+1)—O—C₂H_(2s)— CN H H nAm C_(n)H_(2n+1) COOC_(m)H_(2m+1) H HnOCCF₂.F.F C_(n)H_(2n+1) OCH₂CF₂H F F

Preferred mixture components are given in Tables A and B.

TABLE A

PYP

PYRP

BCH

CBC

CCH

CCP

CPTP

CEPTP

ECCP

CECP

EPCH

PCH

PTP

BECH

EBCH

CPC

B

FET-nF

CGG

CGU

CUP

CCGU

TABLE B

CBC-nmF

PCH-nOm FET-nCl

CP-nOCF₃ CCH-nOm

BCH-n.Fm

Inm

CBC-nmF

ECCP-nm CCH-n1EM

T-nFm CGU-n-F

CCP-nOCF₃.F CGG-n-F

CCP-nOCF₂.F(.F) CCP-nF.F.F

CGU-n-OXF CUZU-n-F

CGU-n-O1DT CCZU-n-F

CC-n-V1 CC-n-V

CCP-nOCF₃ BCH-nF.F.F

CWCQU-n-F

CCOC-n-m

CGZU-n-F CUZP-n-F

CGU-1V-F CCG-V-F

CGZP-n-F UZP-n-N

CGZP-n-OT

CUZP-n-OT

CCQU-n-F Dec-U-n-F

Nap-U-n-F

CQGZP-n-F

CCQP-n-S

CPUQU-n-F

CCEEU-n-F CEECU-n-F

IS-5501 IS-5643

IS-5930 IS-5570

IS-5364 IS-5083

IS-5840

TABLE C Table C indicates possible dopants which are generally added tothe mixtures according to the invention.

C 15

CB 15

CM 21

R/S-811

CM 44

CM 45

CM 47

CN

R/S-4011

R/S-2011

TABLE D Stabilisers which can be added, for example, to the mixturesaccording to the invention are shown below.

The following examples are intended to explain the invention withoutlimiting it. Above and below, percentages are percent by weight. Alltemperatures are indicated in degrees Celsius. m.p. denotes meltingpoint, cl.p. indicates clearing point. Furthermore, C=crystalline state,N=nematic phase, S=smectic phase and I=isotropic phase. The data betweenthese symbols represent the transition temperatures. An denotes opticalanisotropy (589 nm, 20° C.). The rotational viscosity γ₁ (mPa·s) wasdetermined at 20° C.

EXAMPLE 1

A liquid-crystalline medium comprising

A liquid-crystalline medium comprising CCZU-2-F 5.00% Clearing point [°C.]: 65.5 CCZU-3-F 14.00% S-N transition [° C.]: <−40 CCZU-5-F 4.00% Δn[589 nm, 20° C.]: 0.0909 BCH-3F.F.F 7.00% γ₁ [mPa · s]: 139 CGU-2-F10.00% V_((10,0,20)) [V]: 1.15 CGU-3-F 10.00% CGU-5-F 5.00% CCP-20CF₃6.00% CCP-30CF₃ 8.00% CCP-2F.F.F 10.00% CCP-3F.F.F 9.00% CC-1V-V1 4.00%IS-5501 8.00%

EXAMPLE 2

A liquid-crystalline medium comprising

A liquid-crystalline medium comprising PCH-5F 3.59% Clearing point [°C.]: 97.2 CCP-20CF2.F.F 19.12% Δε [1 kHz, 20° C.]: +8.2 CCP-30CF2.F.F17.95% V_((10,0,20)) [V]: 1.38 CCP-50CF2.F.F 19.12% CUP-2F.F 6.01%CUP-3F.F 6.01% CBC-33F 6.01% CBC-53F 6.01% CBC-55F 5.92% IS-5501 10.24%

EXAMPLE 3

A liquid-crystalline medium comprising

A liquid-crystalline medium comprising CCZU-2-F 5.00% Clearing point [°C.]: 70.2 CCZU-3-F 15.00% CCZU-4-F 5.00% CGU-2-F 10.00% CGU-3-F 10.00%CGU-5-F 6.00% BCH-3F.F.F 5.00% CCP-2F.F.F 10.00% CCP-20CF₃ 9.00%CCZG-2-OT 10.00% CCZG-3-OT 5.00% CC-3-V1 4.00% IS-5501 6.00%

EXAMPLE 4

A liquid-crystalline medium comprising

A liquid-crystalline medium comprising PCH-5F 8.50% Clearing point [°C.]: 83.0 PCH-6F 6.80% Δε [1 kHz, 20° C.]: +6.0 PCH-7F 5.10% K₁ [pN, 20°C.]: 12.2 CCP-20CF₃ 6.80% K₃/K₁: 1.41 CCP-30CF₃ 10.20% CCP-40CF₃ 6.00%CCP-50CF₃ 9.30% BCH-3F.F 10.30% BCH-5F.F 8.50% ECCP-30CF₃ 4.20%ECCP-50CF₃ 4.20% CBC-33F 1.70% CBC-53F 1.70% CBC-55F 1.70% IS-5501 5.00%IS-5643 5.50% IS-5364 4.50%

EXAMPLE 5

A liquid-crystalline medium comprising

A liquid-crystalline medium comprising PCH-5F 9.00% Clearing point [°C.]: 87.0 PCH-6F 7.20% Δn [589 nm, 20° C.]: 0.0953 PCH-7F 5.40% Δε [1kHz, 20° C.]: +5.8 CCP-20CF₃ 7.20% CCP-30CF₃ 10.80% CCP-40CF₃ 6.30%CCP-50CF₃ 9.90% BCH-3F.F 10.80% BCH-5F.F 9.00% ECCP-30CF₃ 4.50%ECCP-50CF₃ 4.50% CBC-33F 1.80% CBC-53F 1.80% CBC-55F 1.80% IS-550110.00%

EXAMPLE 6

A liquid-crystalline medium comprising PCH-7F 6.00% Clearing point [°C.]: 93.0 CCP-20CF₃ 11.00% Δn [589 nm, 20° C.]: 0.0943 CCP-30CF₃ 12.00%V_((10,0,20)) [V]: 1.55 CCP-40CF₃ 10.00% CCP-50CF₃ 12.00% BCH-3F.F.F12.00% BCH-5F.F.F 11.00% CCP-3F.F.F 12.00% CCP-5F.F.F 9.00% IS-55015.00%

EXAMPLE 7

A liquid-crystalline medium comprising

A liquid-crystalline medium comprising CCP-20CF₃ 11.00% Clearing point[° C.]: 98.0 CCP-30CF₃ 12.00% Δn [589 nm, 20° C.]: 0.0983 CCP-40CF₃10.00% V_((10,0,20)) [V]: 1.50 CCP-50CF₃ 12.00% BCH-3F.F.F 12.00%BCH-5F.F.F 11.00% CCP-3F.F.F 12.00% CCP-5F.F.F 9.00% IS-5501 11.00%

EXAMPLE 8

A liquid-crystalline medium comprising

A liquid-crystalline medium comprising CCP-20CF₃ 11.00% Clearing point[° C.]: 71.4 CCP-30CF₃ 12.00% CCP-40CF₃ 10.00% CCP-50CF₃ 12.00%BCH-3F.F.F 12.00% BCH-5F.F.F 11.00% CCP-3F.F.F 12.00% CCP-5F.F.F 9.00%IS-5501 5.00% IS-5364 6.00%

EXAMPLE 9

A liquid-crystalline medium comprising

A liquid-crystalline medium comprising PCH-5F 9.00% Clearing point [°C.]: 87.0 PCH-6F 7.20% Δn [589 nm, 20° C.]: 0.0951 PCH-7F 5.40% Δε [1kHz, 20° C.]: +5.7 CCP-20CF₃ 7.20% γ₁ [mPa · s, 20° C.]: 121 CCP-30CF₃10.80% CCP-40CF₃ 6.30% CCP-50CF₃ 9.90% BCH-3F.F 10.80% BCH-5F.F 9.00%ECCP-30CF₃ 4.50% ECCP-50CF₃ 4.50% CBC-33F 1.80% CBC-53F 1.80% CBC-55F1.80% IS-5364 10.00%

EXAMPLE 10

A liquid-crystalline medium comprising

A liquid-crystalline medium comprising PCH-5F 5.00% Clearing point [°C.]: 90.8 CCP-20CF₃ 11.00% CCP-30CF₃ 12.00% CCP-40CF₃ 10.00% CCP-50CF₃12.00% BCH-3F.F.F 12.00% BCH-5F.F.F 11.00% CCP-3F.F.F 12.00% CCP-5F.F.F9.00% IS-5364 6.00%

EXAMPLE 11

CC-3-V1 10.00% Clearing point [° C.]: +80.0 CC-5-V 10.00% Δn [589 nm,20° C.]: +0.1033 CCH-35 5.00% d · Δn [μm, 20° C.]: 0.50 IS-5501 8.00%Twist [°]: 90 IS-5643 4.00% V_(10,0,20) [V]: 1.37 CCP-20CF₃ 8.00%CCP-30CF₃ 3.00% CCP-2F.F.F 3.00% PGU-2-F 8.00% PGU-3-F 8.00% CGZP-2-OT11.00% CGZP-3-OT 9.00% CCZU-2-F 4.00% CCZU-3-F 9.00%

EXAMPLE 12

CCH-35 4.00% S → N [° C.]: <−40.0 IS-5501 9.00% Clearing point [° C.]:+78.5 CCP-2F.F.F 10.00% Δn [589 nm, 20° C.]: +0.1033 CCP-3F.F.F 11.00% d· Δn [μm, 20° C.]: 0.50 CCP-20CF₃.F 9.00% Twist [°]: 90 CCP-20CF₃ 8.00%CCP-30CF₃ 8.00% CCP-40CF₃ 7.00% CCP-50CF₃ 4.00% CGU-2-F 10.00% CGU-3-F11.00% CGU-5-F 4.00% CCGU-3-F 5.00%

EXAMPLE 13

PCH-3 22.50% Clearing point [° C.]: +111.0 K6 7.20% Δε: +9.8 K9 8.10% K₁[pN]: 14.8 CCP-20CF₃ 4.50% K₃/K₁: 1.55 CCP-30CF₃ 4.50% CCP-40CF₃ 4.50%CCP-50CF₃ 4.50% ECCP-20CF₃ 4.50% ECCP-30CF₃ 4.50% ECCP-50CF₃ 4.50%ECCP-3F 4.50% ECCP-5F 4.50% CBC-33F 4.50% CBC-53F 3.60% CBC-55F 3.60%IS-5840 10.00%

EXAMPLE 14

PCH-6F 7.20% Clearing point [° C.]: +93.0 PCH-7F 5.40% γ₁: [20° C., mPa· s]: 140 CCP-20CF₃ 7.20% CCP-30CF₃ 10.80% CCP-40CF₃ 6.30% PCH-5F 9.00%CCP-50CF₃ 9.90% BCH-3F.F 10.80% BCH-5F.F 9.00% ECCP-30CF₃ 4.50%ECCP-50CF₃ 4.50% CBC-33F 1.80% CBC-53F 1.80% CBCF-55F 1.80% IS-584010.01%

EXAMPLE 15

PCH-3 22.49%  Clearing point [° C.]: +100.6 K6 7.20% Δε: +9.8 K9 8.10%CCP-20CF₃ 4.50% CCP-30CF₃ 4.50% CCP-40CF₃ 4.50% CCP-50CF₃ 4.50%ECCP-20CF₃ 4.50% ECCP-30CF₃ 4.50% ECCP-50CF₃ 4.50% ECCP-3F 4.50% ECCP-5F4.50% CBC-33F 4.50% CBC-53F 3.60% CBC-55F 3.60% IS-5840 10.03% 

EXAMPLE 16

BCH-3F.F 10.80%  Clearing point [° C.]: +65.8 BCH-5F.F 9.00% γ₁ [20° C.,mPa · s]: 91 ECCP-30CF₃ 4.50% d · Δn [20° C., μm]: 0.50 ECCP-50CF₃ 4.50%Twist [°]: 90 CBC-33F 1.80% CBC-53F 1.80% CBC-55F 1.80% PCH-6F 7.20%PCH-7F 5.40% CCP-20CF₃ 7.20% CCP-30CF₃ 10.80%  CCP-40CF₃ 6.30% CCP-50CF₃9.90% PCH-5F 9.00% IS-5083 9.97%

EXAMPLE 17

CCZU-2-F 3.00% Clearing point [° C.]: +70.5 CCZU-3-F 13.00%  Δn: +0.0772CCP-20CF₃ 4.00% γ₁ [20° C., mPa · s]: 54 CCP-30CF₃ 8.00% d · Δn [20° C.,μm]: 0.50 CGZP-2-OT 7.00% Twist [°]: 90 CGZP-3-OT 6.00% V₁₀ [V]: 1.73PGU-2-F 5.00% IS-5083 7.00% CC-5-V 10.00%  CC-3-V1 12.00%  CCH-35 5.00%CC-3-V 18.00%  BCH-32 2.00%

EXAMPLE 18

CCZU-2-F 3.00% Clearing point [° C.]: +74.5 CCZU-3-F 13.00%  Δn: +0.0776CCP-20CF₃ 7.00% γ₁ [20° C., mPa · s]: 57 CCP-30CF₃ 6.00% d · Δn [20° C.,μm]: 0.50 CGZP-2-OT 8.00% Twist [°]: 90 CGZP-3-OT 7.00% V₁₀ [V]: 1.81PGU-2-F 3.00% IS-5083 6.00% CC-5-V 7.00% CC-3-V1 13.00%  CCH-35 5.00%CC-3-V 19.00%  BCH-32 3.00%

EXAMPLE 19

CCZU-2-F 3.00% CZU-3-F 13.00% CCP-20CF₃ 7.00% CGZP-2-OT 7.00% CGZP-3-OT6.00% PGU-2-F 4.00% IS-5083 6.00% CC-5-V 10.00% CC-3-V1 11.00% CCH-355.00% CC-3-V 18.00% CBC-33 3.00%

EXAMPLE 20

PCH-3 20.00% K6 6.40% K9 7.20% CCP-20CF₃ 4.00% CCP-30CF₃ 4.00% CCP-40CF₃4.00% CCP-50CF₃ 4.00% ECCP-20CF₃ 4.00% ECCP-30CF₃ 4.00% ECCP-50CF₃ 4.00%ECCP-3F 4.00% ECCP-5F 4.00% CBC-33F 4.00% CBC-53F 3.20% CBC-55F 3.20%IS-5930 20.00%

EXAMPLE 21

PCH-6F 7.60% Clearing point [° C.]: +91.1 PCH-7F 5.70% γ₁ [20° C., mPa ·s]: 141 CCP-20CF₃ 7.60% CCP-30CF₃ 11.40% CCP-40CF₃ 6.65% PCH-5F 9.50%CCP-50CF₃ 10.45% BCH-3F.F 11.40% BCH-5F.F 9.50% ECCP-30CF₃ 4.75%ECCP-50CF₃ 4.75% CBC-33F 1.90% CBC-53F 1.90% CBC-55F 1.90% IS-5930 5.01%

EXAMPLE 22

IS-5570 20.00% PCH-5F 3.20% CCP-30CF₂.F.F 16.00% CCP-50CF₂.F.F 17.04%CUP-2F.F 5.36% CUP-3F.F 5.36% CBC-33F 5.36% CBC-53F 5.36% CBC-55F 5.28%CCP-20CF₂.F.F 17.04%

1. Liquid-crystalline medium based on a mixture of polar compounds ofpositive dielectric anisotropy, which comprises: one or more compoundsof the formula I

in which R is F, Cl, Br, I, CN, NCS, SF₅ or an alkyl radical having from1 to 12 carbon atoms which is unsubstituted, monosubstituted by CN orCF₃ or monosubstituted or polysubstituted by halogen and in which one ortwo non-adjacent CH₂ groups are optionally replaced by —O—, —CH═CH—,—CF═CF—, —C≡C—, —CO—, —OCO— or —COO— in such a way that O atoms are notlinked directly to one another, A¹ and A² are each, independently of oneanother, 1,4-phenylene, in which one or two CH groups are optionallyreplaced by N and which is optionally monosubstituted or polysubstitutedby L, or are trans-1,4-cyclohexylene, in which one or more non-adjacentCH₂ groups are optionally replaced by —O— and/or —S—, or are1,4-cyclohexenylene, 1,4-bicyclo[2.2.2]octylene, piperidine-1,4-diyl,naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl or1,2,3,4-tetrahydronaphthalene-2,6-diyl, L is F, Cl, Br, I, CN, NCS, SF₅or alkyl, alkoxy, alkylcarbonyl, alkylcarbonyloxy, alkoxycarbonyl,alkenyl or oxaalkenyl having from 1 to 3 carbon atoms, in which one ormore H atoms are optionally replaced by F or Cl, Z¹ and Z² are each,independently of one another, —CH₂O—, —OCH₂—, —CF₂O—, —OCF₂—, —COO—,—OCO—, —CF₂CF₂—, —CH₂CH₂—, —(CH₂)₄—, —(CH₂)₃O—, —O(CH₂)₃—, —CF₂CH₂—,—CH═CH—, —CH═CF—, —CF═CF—, —C≡C— or a single bond, X¹ and X² are each,independently of one another, F, Cl, Br, I, CN, NCS, SF₅ or alkyl,alkoxy, alkylcarbonyl, alkylcarbonyloxy, alkoxycarbonyl, alkenyl oroxaalkenyl having 1 to 5 carbon atoms, in which one or more H atoms areoptionally replaced by F or Cl, and one of the radicals X¹ and X² isalternatively H or R, and n and o are each, independently of oneanother, 0, 1 or 2, where n+o is ≦3 and, one or more compounds selectedfrom the group consisting of the compounds of formulae II, III, IV, V,VI and VII:

in which the individuals have the following meanings: R⁰ is n-alkyl,alkoxy, fluoroalkyl, alkenyl or oxaalkenyl, each having 1 to 9 carbonatoms, Z³ is —COO—, —CF₂O—, —C₂F₄— or a single bond, Z⁴ is —COO—,—CF₂O—, —C₂H₄—, X⁰ is F, Cl, halogenated alkyl, alkenyl or alkoxy havingfrom 1 to 6 carbon atoms. Y¹ and Y² are each, independently of oneanother, H or F, and r is 0 or
 1. 2. Liquid-crystalline medium accordingto claim 1, which comprises one or more compounds of the formulae Ia toIe:

in which R, Z1, X1 and X2 are as defined in claim
 1. 3.Liquid-crystalline medium according to claim 1, wherein, in the formulaeI, n+o is 0 or
 1. 4. Liquid-crystalline medium according to claim 1,wherein, in the formula I, X¹ and/or X² are F, Cl, CN, fluorinated alkylor fluorinated alkoxy, each having from 1 to 3 carbon atoms. 5.Liquid-crystalline medium according to claim 1, which additionallycomprises one or more compounds selected from the group consisting ofthe compounds of formulae VIII to XIV:

in which R⁰, X⁰, Y¹ and Y² are as defined in claim
 1. 6.Liquid-crystalline medium according to claim 1, which additionallycomprises one or more compounds selected from the group consisting ofthe compounds of formulae XXI to XXVII:

in which R⁰ and Y¹ are as defined in claim 1, and R¹ and R² are each,independently of one another, alkyl or alkoxy having from 1 to 8 carbonatoms or alkenyl having from 2 to 7 carbon atoms.
 7. Liquid-crystallinemedium according to claim 1, wherein the proportion of compounds of theformula I in the mixture as a whole is from 2 to 55% by weight. 8.Liquid-crystalline medium according claim 1, wherein the proportion ofcompounds of the formulae I to VII in the mixture as a whole is at least50% by weight.
 9. Electro-optical liquid-crystal display containing aliquid-crystalline medium according to claim
 1. 10. Liquid-crystallinemedium based on a mixture of polar compounds of positive dielectricanisotropy, which comprises: one or more compounds of the formula I

in which R is F, Cl, Br, I, CN, NCS, SF₅ or an alkyl radical having from1 to 12 carbon atoms which is unsubstituted, monosubstituted by CN orCF₃ or monosubstituted or polysubstituted by halogen and in which one ortwo non-adjacent CH₂ groups are optionally replaced by —O—, —CH═CH—,—CF═CF—, —C≡C—, —CO—, —OCO— or —COO— in such a way that 0.0 atoms arenot linked directly to one another, A¹ and A² are each, independently ofone another, 1,4-phenylene, in which one or two CH groups are optionallyreplaced by N and which is optionally monosubstituted or polysubstitutedby L, or are trans-1,4-cyclohexylene, in which one or more non-adjacentCH₂ groups are optionally replaced by —O— and/or —S—, or are1,4-cyclohexenylene, 1,4-bicyclo-[2.2.2]octylene, piperidine-1,4-diyl,naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl or1,2,3,4-tetrahydronaphthalene-2,6-diyl, L is F, Cl, Br, I, CN, NCS, SF₅or alkyl, alkoxy, alkylcarbonyl, alkylcarbonyloxy, alkoxycarbonyl,alkenyl or oxaalkenyl having from 1 to 3 carbon atoms, in which one ormore H atoms are optionally replaced by F or Cl, Z¹ and Z² are each,independently of one another, —CH₂O—, —OCH₂—, —CF₂O—, —OCF₂—, —COO—,—OCO—, —CF₂CF₂—, —CH₂CH₂—, —(CH₂)₄—, —(CH₂)₃O—, —O(CH₂)₃—, —CF₂CH₂—,—CH═CH—, —CH═CF—, —CF═CF—, —C≡C— or a single bond, X¹ and X² are each,independently of one another, F, Cl, Br, I, CN, NCS, SF₅ or alkyl,alkoxy, alkylcarbonyl, alkylcarbonyloxy, alkoxycarbonyl, alkenyl oroxaalkenyl having 1 to 5 carbon atoms, in which one or more H atoms areoptionally replaced by F or Cl, and one of the radicals X¹ and X² isalternatively H or R, and n and o are each, independently of oneanother, 0, 1 or 2, where n+o is <3; and, one or more compounds selectedfrom the group consisting of the compounds of formulae VIII to XIV:

in which R⁰ is n-alkyl, alkoxy, fluoroalkyl, alkenyl or oxaalkenyl, eachhaving 1 to 9 carbon atoms, X⁰ is F, Cl, halogenated alkyl, alkenyl oralkoxy having from 1 to 6 carbon atoms, Y¹ and Y² are each,independently of one another, H or F.
 11. Liquid-crystalline mediumaccording to claim 10, which comprises one or more compounds of theformulae Ia to Ie:

in which R, Z¹, X¹ and X² are as defined in claim
 10. 12.Liquid-crystalline medium according to claim 10, wherein, in theformulae I, n+o is 0 or
 1. 13. Liquid-crystalline medium according toclaim 10, wherein, in the formula I, X¹ and/or X² are F, Cl, CN,fluorinated alkyl or fluorinated alkoxy, each having from 1 to 3 carbonatoms.
 14. Liquid-crystalline medium according to claim 10, whichadditionally comprises one or more compounds selected from the groupconsisting of the compounds of formulae XXI to XXVII:

in which R⁰ and Y¹ are as defined in claim and R¹ and R² are each,independently of one another, alkyl or alkoxy having from 1 to 8 carbonatoms or alkenyl having from 2 to 7 carbon atoms.
 15. Liquid-crystallinemedium according to claim 10, wherein the proportion of compounds of theformula T in the mixture as a whole is from 2 to 55% by weight. 16.Electro-optical liquid-crystal display containing a liquid-crystallinemedium according to claim
 10. 17. An electro-optical liquid-crystaldisplay of claim 9, wherein the display is a MLC, TN, STN, OCB or IPSdisplay.
 18. An electro-optical liquid-crystal display of claim 16,wherein the display is a MLC, TN, STN, OCB or IPS display. 19.Liquid-crystalline medium according to claim 1, wherein the mediumretains a nematic phase down to −20° C., has a clearing point above 75°C., and has a dielectric anisotropy of ≧5.
 20. Liquid-crystalline mediumaccording to claim 10, wherein the medium retains a nematic phase downto −20° C., has a clearing point above 75° C., and has a dielectricanisotropy of ≧5.